The present invention relates to an apparatus and system for altering the frequency of a Fourdrinier table in the formation of a continuous web of paper or other material.
In the manufacture of paper, a stock of fibers and mineral fillers suspended in water, is deposited onto the moving wire on the Fourdrinier table of a paper machine. An example of a conventional Fourdrinier table assembly 10 is shown in FIG. 1. The table 10 includes a head box 12 from which a stock suspension is deposited onto a continuously moving wire 14, a breast roll 16, forming unit 18, and a series of gravity foil boxes 20 and vacuum foil boxes 22, a dandy roll 24, a series of suction boxes 26, and a couch roll 28. As the stock suspension moves along the wire 14 and over the foil boxes 20, 22 and suction boxes 26, the water is removed to form a continuous web.
Many theories have been applied to enhance water removal and achieve proper fiber orientation and distribution to form the fiber sheet, but with varying degrees of success. In one practice, table rolls have been used to apply a vacuum pulse by drawing water from the undersurface of the wire, and then create a pressure pulse by pushing water through the fabric to agitate the stock suspension for proper fiber orientation. However, as production speeds increased and higher vacuum forces were applied, excessive jumping of the stock of the forming sheet occurred which adversely affected formation quality. With the development of hydrofoils, control of water removal and formation improved.
From 1960 to 1970, machines became faster and wider, and the gravity foil box was introduced. The device consisted of a bridge-like framework that spanned the table with xe2x80x9cTxe2x80x9d bars installed for the individual blades. Foil blades could be removed or added on the run, and the spacing of the xe2x80x9cfoil banksxe2x80x9d was random at best. The concept of foil angle was then proposed and experimentation was performed to determine optimal foil blade angle and foil bank spacing on the machine, which are important to drainage and formation.
A subsequent development was the concept of table harmonics, an engineering principle stating that the energy contained within the stock at the exit of the head box can be amplified (for improved drainage and formation) by the spacing of the foils. The harmonic excitation of the stock can be further altered by placing foil banks at specific intervals along the table based on the tip-to-tip spacing of the foils within each bank. This principle gave rise to the practice of placing the start of a first foil bank in the vicinity of three to six feet from the exit of the head box. It was also learned that the ability to add or remove foils from a bank significantly impacted sheet properties. However, foil banks could not be moved while the machine was running due to the tremendous drag imparted onto the foils.
In about 1978, the concept of table frequency was combined with table harmonics to maximize drainage and formation. It was discovered that packing a table with foils spaced an appropriate distance apart, and then removing the foils from the table in strategic locations, achieved the desired Fourdrinier frequency when operating at higher speeds, up to 3300 fpm and higher.
Another development included the introduction of an automated foil bank that varied the pitch of the foil blade (the variable angle foil) to impact drainage and formation. It was also determined that the best formation and drainage for any given table was a frequency between 55 Hz and 105 Hz. In addition, a foil bank system was introduced that could raise foils into the wire and/or drop them from contact with the wire, but only allowed the use of a finite number of frequencies (i.e., either 55 or 75 Hz) by the papermaker. This limits the success of the papermaker where another frequency (i.e., 61 Hz) would be optimal for formation and drainage.
The function of the Fourdrinier table is two-fold: (1) to de-water the stock utilizing the effects of both gravity and applied vacuum, and (2) to subject the stock to periodic excitation as the wire passes over a series of inverted continuous hydrofoil blades (foils) that extend transversely across the table in a cross machine direction, i.e., at a right angle to the direction in which the wire travels.
Traditionally, a Fourdrinier table include several sections of foil groupings, or sets, of approximately six foils each, that are mounted on individual foil support beam structures (i.e., T-bar mounts) spaced along the length of the table at set intervals to create a desired pulse frequency. The foil sets are normally affixed to a sub-structure of the table commonly referred to as a xe2x80x9cbox.xe2x80x9d An example of a conventional foil box 30 having four foils 34 is shown in FIG. 2. The direction of the movement of the wire (not shown) over the foils 34 is shown by arrow 30. The boxes are further sub-classified into either gravity boxes 20 or vacuum boxes 22 (FIG. 1). The first several foil sets aid in de-watering the stock under the influence of gravity. Further down the table as the water content of the stock decreases, a vacuum is applied from beneath the wire to facilitate the de-watering process.
The foils aid in the de-watering process and also impart a pressure impulse to the stock suspension. The impulses serve to keep the fibers and fillers in suspension during the de-watering process yielding a paper stock of uniform consistency. A single pulse is not adequate to control the stock on the Fourdrinier table. Rather, a series of pulses is generated and repeated at a standard interval.
The frequency of these impulses is referred to as the Fourdrinier frequency, which is defined as the velocity of the wire (in inches-per-second) divided by the pitch distance between the foils (in inches). It is well known to those versed in the art/science of papermaking that the frequency of these impulses has a dramatic effect upon the formation of the paper fibers. Under most circumstances, acceptable formation occurs at a Fourdrinier frequency between about 55 hertz and about 90 hertz. However, the current state of the art/science of paper formation relies upon the strategic use of conventional foil blades, multi-pulse foils, and/or foil boards that compromise effective stock de-watering with appropriate stock excitation frequencies.
The present invention provides variable frequency foil (VFF) box assemblies and mechanisms for moving individual foils/foil beams and individual foil beam sets relative to each other to adjust the frequency of a paper making machine independent of the wire speed. The invention allows for continuously and uniformly adjusting the pitch distances of individual foils within foil sets over a finite range, and also adjusting the distance between foil sets during the operation of a paper making machine.
In one aspect, the invention provides a foil beam assembly. In one embodiment, the foil beam assembly comprises at least a first and a second foil beam set, each foil beam set comprising a leading foil beam, a trailing foil beam, and at least one intermediate foil beam disposed therebetween, and a mechanism to laterally move the foil beams and the foil sets relative to each other. The mechanism is connected to each of the foil beams and to the first and second foil beam set. The mechanism is operable to laterally move the foil beams to alter the pitch distance such that each of the foil beams are spaced apart by a standard interval, and to laterally move at least one of the foil beam sets to alter the distance therebetween such that the foil beam sets are spaced apart by an integer multiple of the standard interval.
In one embodiment of the foil beam assembly, the mechanism can comprise a mating screw and nut assembly affixed to a first foil beam and an adjacent second foil beam, and in rotatable contact with a gear mounted on a shaft, whereby rotating the shaft causes lateral movement of at least the second foil beam to alter the pitch distance between the first and second foil beams. In another embodiment, the mechanism of the foil beam assembly comprises a hydraulic or pneumatic device mounted on the first and second foil beams and operable to laterally move at least the second foil beam relative to the first foil beam. In another embodiment of the foil beam assembly, the mechanism can comprise an activating screw and nut assembly affixed to the second foil beam and oriented perpendicular to the foil beams, the activating screw connected to an actuating device operable to move the activating screw to laterally move the second foil beam relative to the first foil beam. In yet another embodiment, the mechanism of the foil beam assembly can comprise nut members mounted on a surface of the first and second foil beams, and activating screw members engaged through the nut members and extending perpendicular to the foil beams, the activating screw members connected to actuators comprising a worm/gear assembly mounted on a drive shaft, wherein movement of the actuators move the activating screw members which laterally move at least the second foil beam relative to the first foil beam. Yet another embodiment of a mechanism for use in the foil beam assembly comprises a pantograph assembly connected to the first and second foil beams, wherein extension and retraction of the pantograph moves at least the second foil beam relative to the first foil beam to alter the pitch distance therebetween. A further embodiment of the mechanism of the foil beam assembly comprises a telescoping shaft assembly.
In another aspect, the invention provides a method of varying the frequency of a foil beam set. In one embodiment, the method comprises the steps of providing at least a first and second foil beam set, each set comprising two or more foil beams mounted on a support structure, and a mechanism interconnecting the foil beams and the foil beam sets, the mechanism structured to laterally move the foil beams relative to each other and to laterally move the foil beam sets relative to each other; and actuating the mechanism to laterally move the foil beams to alter the distance therebetween and maintain the foil beams at a distance X relative to each other, and to laterally move the foil beam sets relative to each other to a distance as an integer multiple of the distance X, wherein the combined frequency of the foil beam sets is maintained at about 50 to about 90 hertz.